Add option to change "go back" icon for AboutPage or MarkdownPage

[dvorapa]

Is your feature request related to a problem? Please describe. My app has got specific "go back" icons in all places. These are the only ones I was unable to change to the same style. Would it be possible to add an option to change also this icon?

Describe the solution you'd like MarkdownPageListTile(returnIcon: Icons.navigate_before) could be the option.

Describe alternatives you've considered I considered to avoid your package at all, but I was unable to recreate your amazing MarkdownPageListTile and MarkdownPage at all!